The invention relates to a method and a device for cooling a preliminary strip or strip of a metal strand in a hot rolling mill which contains work rolls, wherein a cooling agent is sprayed onto the preliminary strip or the strip.
In a hot rolling mill, several cooling devices are known or defined in the areas of the preliminary train and the finishing train, such as:                the preliminary strip cooling (arrangement between preliminary stand and first finishing stand)        the intermediate stand cooling (arranged between two finishing stands)        the strip bottom side cooling (positioned in front of a lateral guidance or behind a loop lifter)        roll gap cooling (cooling of the strip shortly before the roll gap on both sides or one side).        
The various cooling devices influence the surface temperature pattern, scale growth, mechanical properties, production quantity, etc. The cooling occurs uniformly across the width of the strip. As a rule, the temperature of the preliminary strip as well as the strip temperature are colder in the entire finishing train in the areas of the strip edges. The above-mentioned cooling devices further enforce this effect. By using the method and devices described in the invention, this disadvantage is to be avoided and the temperature uniformity is to be even improved. The novel cooling method or cooling device will be described in more detail in connection with the example of the roll gap cooling.
A device of this type is known from patent DE 4134599 C1. This patent relates to a method for hot rolling of metal strip with one or more passes, wherein especially on the inlet side of the roll stand or the roll stands the preliminary strip or the strip are cooled.
The known method is characterized in that the spraying of the cooling liquid on the inlet side over the entire width of the metal strip takes place as closely as possible in front of the roll gap onto the metal strip and onto the jackets of the work rolls, so that the strip surface is undercooled and thereby a low thermal flux into the work roll is created.
This manner of operation is based on the recognition that by cooling the strip surface of a steel strip immediately prior to entering the roll gap formed by two rolls the service life of the roll surface is increased to a significant extent. As a result of this measure a “peeling” of the work rolls, i.e., a separation of the oxide layer from the upper surface of the work roll, is substantially avoided or significantly postponed. This also has an indirect positive effect on the quality of the strip surface.
As a rule, the temperature of the preliminary strip as well as of the strip of a metal strip to be rolled is lower in the middle of the preliminary strip or of the strip. In FIG. 1, the temperature of a metal strip is illustrated as a function of the width of the metal strip which travels through the roll stands of a hot rolling mill. In this drawing, the strip middle of a metal strip is illustrated at the left border of the diagram. A curve 1 shows the pattern of the temperature prior to the entry of the metal strip into a roll stand. A curve 2 shows the temperature pattern after the emergence from the roll stand. In both curves, the sharp temperature drop in the edge areas of the metal strip can be recognized. Due to the fact that the temperature of the metal strip, as well as of the preliminary metal strip, is lower in the edge areas than in the middle, different local loads act on the metal strip which themselves have a negative effect on the metal strip as well as on the work rolls. The affected areas of the work rolls may wear to a greater extent, so that profile anomalies and/or problems with respect to strip planeness occur. The cold strip edges can also have a negative effect on the metallurgical properties at the edges of the strip.
The use of a method known from the prior art for cooling the middle of the strip prior to the roll gap can even further increase the temperature difference of the strip toward the strip edges and the associated negative effects. Similar effects also occur in the other cooling devices mentioned above.
U.S. Pat. No. 5,235,840 discloses a device for processing a steel strip in the hot rolling plant and a method for minimizing scale growth on the steel strip, as well as for reducing the wear of the work rolls in the finishing train. In order to reduce the scale growth or oxidation of the steel strip, the steel strip is sprayed within the finishing rolling train at selected locations on the strip with cooling agent, wherein the surface temperature of the strip is controlled or regulated in such a way that it is within a temperature range within which the scale growth or the oxidation is minimized. This automatically minimizes the wear of the work rolls.
European Patent Application EP 1 634 657 A1 discloses a system for controlling or regulating the cooling agent application on a steel plate after the plate has been hot and cold rolled. For this purpose, the temperature distribution on the steel strip and steel plate is simplified in the width direction prior to the beginning of the cooling process and the steel plate is then cooled by means of a cooling regulation with a cooling rate which is equal over the width.
German Patent DE 32 30 866 discloses a device for cooling a steel sheet plate immediately after hot rolling. In the area of the side edges of the steel sheet plate, screening members are arranged which can be moved in the width direction of the steel sheet plate which is placed on a roller conveyor toward each other and away from each other by means of a displacement device.
Japanese Patent Application JP 59078710 discloses a method for controlling or regulating the temperature of a finished-rolled steel strip, wherein screening devices are provided at opposite located edges of the steel strip, wherein the screening devices prevent the impingement of cooling agent on the edges of the steel strip. The screening devices can be moved toward each other and away from each other and can in this manner be adjusted to the width of the steel strip. In addition, the area of the middle of the steel strip can be defined which is to be sprayed with the cooling agent.